In recent years, the modern sporting rifle (MSR) has become a popular firearm for use in hunting and target practice. The MSR is based on the AR-15 platform designed by Eugene Stoner while working as an engineer at the Armalite Company. The MSR may sometimes appear cosmetically similar to military rifles, such as the M-16. However, the MSR functions like other semi-automatic civilian sporting rifles, firing only one round with each pull of the trigger. The MSR is commercially available from several manufacturers. Each manufacturer may offer several MSR models in popular configurations.
The MSR is a gas operated rifles utilizing either a direct gas impingement system for operating their ejection and loading mechanisms. The expanding gas from the cartridge propellant is tapped from a port in the barrel intermediate the chamber and the muzzle end of the barrel. In the direct gas impingement system, a conduit extends from the port to the upper receiver and into the region of the bolt carrier. During the initial firing of the cartridge, the bolt is locked into the barrel extension, the gas forces the bolt carrier backward a short distance to unlock the bolt. As the bolt carrier moves toward the butt of the gun, a bolt cam pin, forces the bolt to rotate, by this time the bullet has left the barrel. The inertia of the bolt and bolt carrier continues the rearward motion causing the bolt to extract the fired empty cartridge. A spring absorbs the rearward motion of the bolt and bolt carrier forcing the bolt and bolt carrier forward to engage the next cartridge in the magazine and push same into the chamber ready for firing.